Create a migration guide for users of constant output functions (MD5, SHA-1)

We should have a migration guide for users of constant-output hashing like SHA1 and MD5.
[`Sel.Hashing`](https://hackage.haskell.org/package/sel-0.0.1.0/candidate/docs/Sel-Hashing.html) is an adequate replacement.

Depends on https://github.com/haskell-cryptography/libsodium-bindings/issues/105